Well that's a new one. Most likely this is because the check mark graphic file is straight-up missing. That would normally not happen, but there has been a rash of weird reports like this. Most have been traced to an anti-virus that is blocking simple files for no good reason. If you run Bitdefender, then that is very likely the culprit. Regardless, check your anti-virus logs to see if it has quarantined any SkyTools files.

Also, try this: open the preferences dialog. Drag it to the side of the screen such that at least some of the check boxes go off the screen and out of view. Then return it to the screen. Did some of the check boxes now draw properly?